1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to a mechanically openable and closable safety cover for the die closing unit of an injection molding machine which has a drive-reversing safety mechanism with a contact member on its leading edge, for the protection of the machine operator.
2. Description of the Prior Art
Injection molding machines require a safety cover, or covers, which prevent access to the injection molding die, when the machine is in operation. The use of a mechanically closable cover brings with it the additional requirement of protecting the operator or mechanic against injury from the closing cover.
The East German Pat. No. 61 894 discloses a drive-reversing safety mechanism on the leading edge of a movable screen which is arranged in front of the die closing unit of an injection molding machine. The safety screen is openable and closable by means of a pneumatic cylinder assembly, the extremity of its piston rod being connected to the frame of the safety screen. Arranged a distance ahead of the leading edge of the safety screen is a vertical contact bar which is spring-biased forwardly, away from the leading edge. The contact bar is supported and guided by the horizontally movable plunger of a pneumatic valve. This valve, when actuated by an approach displacement of the contact bar towards the leading edge of the safety screen, redirects the air which is supplied to the pneumatic cylinder, thereby reversing the movement of the latter and reopening the safety gate.
The disclosed contact member is guided for straight-line motion by virtue of its attachment to the valve plunger. Such a straight-line guide, in order to be effective, requires guide points which are spaced far apart, as well as correspondingly sturdy components, in order to prevent canting from contact forces which are vertically offset from the guide axis and/or engage the contact bar from an angle relative to its guide axis.
Another shortcoming of this prior art device relates to the relatively elevated force which is required to displace the safety bar and its attached pneumatic valve plunger, in opposition to preloaded compression springs which have to be strong enough to move the safety bar and plunger in the opposite direction. This problem is aggravated by a relatively slow response of the pneumatic cylinder to the action of the pressure-reversing valve.